drm/i915: Handle inactivating objects for all VMAs

This came from a patch called, "drm/i915: Move active to vma"

When moving an object to the inactive list, we do it for all VMs for
which the object is bound.

The primary difference from that patch is this time around we don't not
track 'active' per vma, but rather by object. Therefore, we only need
one unref.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>